The present invention relates to permanently plasticized shaped articles, preferably packaging material, such as films, in particular, tubings, comprising a permanently plasticized cellulose hydrate composition which comprises a mixture of cellulose hydrate and a permanently plasticizing ester. The mixture may further contain water, and if desired, a secondary chemical plasticizer. The present invention, in particular, relates to a tubular casing comprising said permanently plasticized cellulose hydrate composition for use as a packaging material, in particular, for use as an artificial sausage casing.
Furthermore, the present invention comprises processes for the manufacture of a shaped article, in particular, a tubing, comprising said permanently plasticized cellulose hydrate composition.
Shaped articles of cellulose hydrate, in particular, packaging material, can be readily disposed of after use, by burning them, and they have the further advantage in that they undergo bacterial decomposition relatively rapidly. It is well known that in the course of time, cellulose hydrate materials, such as films and casings, change their properties, in particular, their shrinking properties. This latter property is particularly important in the case of sausage casings, since it is necessary that the sausage casing always adapts itself to the likewise shrinking sausage material in such a manner that the sausage casing is always well filled with the sausage composition. Tubular casings of synthetic polymers, such as polyamide, polyester, polyvinylidene chloride or the like, do not show this advantageous behavior to the required degree, due to their inert properties. For this reason, sausages with casings of the above polymers frequently become wrinkled and unattractive in an undesired manner and thus unsellable.
The properties and the structure of the cellulose hydrate forming the shaped articles can be varied within wide limits by varying the properties and composition of the starting viscose solution, the average degree of polymerization, the spinning conditions, stretching conditions, and drying conditions, during the manufacture of the shaped articles and the plasticizer content thereof.
It has also been proposed to improve the suppleness of plasticizer-free cellulose hydrate by adding fat-like chemical substances which are capable of crosslinking.
In the case of synthetic water vapor impermeable sausage casings which are made of a cellulose hydrate material which has been plasticized with a secondary chemical plasticizer and which are coated on their inside with a water vapor impermeable polymer film, extensive shrinkage of the casing occurs after removal or loss of the secondary plasticizer. Such removal of the secondary plasticizer from the casing material is unavoidable during the manufacture of sausages. This shrinkage causes a high internal pressure coupled with a simultaneous embrittlement of the cellulose hydrate. Thus, there is a risk that these casings, in most cases, will tear over the entire length of the casing when the sausage is cut.
If the tearing of the casings is prevented by previously moistening or cooling the sausage, a considerable part of the sausage material will be pressed out from the cut surface in an undesirable manner as a result of the high internal pressure.
During the course of the manufacturing process, the initial shaped bodies, which are in the gel state, may be subjected to the action of a secondary chemical plasticizer. The nature and amount of the plasticizer provides a further possibility of structurally influencing the structure and properties of the resulting cellulose hydrate material.
It is undesirable that during the storage of the shaped articles, the structure of the cellulose hydrate changes as a function of time, thereby causing a deterioration in the mechanical properties of the shaped articles.
The cellulose hydrate is densified by crystallization due to the formation of hydrogen bridge bonds, thus shrinks correspondingly and displaces a part of the secondary plasticizer from the fabric. Although the tensile strength of the shaped article then rises, its extensibility and its swelling value decrease.
Even more serious and disadvantageous are the changes in structure and properties, which take place on quantitative removal of the secondary plasticizer from the shaped article, which takes place if the latter is subjected to the action of water during its intended use. In this event, the cellulose hydrate shrinks very strongly, is densified and is embrittled. Admittedly, this process has the advantage that, when tubing of this type is used as artificial sausage casings, the material filled in it is always tightly enclosed by the latter, but it also has the disadvantage that the shrinkage is more extensive than necessary, so that the internal pressure in the sausage becomes excessive, the embrittled sausage casing then frequently tears when the sausages are cut open, and the sausage casings burst during manufacture or transportation of the sausages.
This undesirable effect occurs in particular, if the sausage casings additionally have a water impermeable coating on their inside.
Although it is known, in order to avoid this disadvantage, to add plasticizing chemical crosslinking agents into the solution or dispersion of the viscose used for the manufacture of the tubing, so that the defects mentioned can be largely eliminated, the known process has the disadvantage that the chemical compounds used are of a complicated structure and their quantitative synthesis is difficult to achieve, that is the resulting products are usually not uniform. Thus, the spinning fluid may, in an uncontrolled manner, contain undesirable side and fission products of the fatty chemical crosslinking agent, which have an unfavorable effect on the quantitative balance between the chemical crosslinking agent and the viscose contained in the spinning fluid and also on the quantitative balance between the viscose and the liquid solvent or dispersing agent used for the chemical crosslinking agent within the spinning fluid.
The use of short-chain crosslinking agents or of high-molecular polyglycols disclosed in the U.S. Pat. No. 3,843,378, either does not have the desired effect or these additives are quantitatively removed from the tubing by re-dissolution during the manufacturing process.
Hitherto it has not been possible to satisfactorily adjust the properties of shaped articles of cellulose hydrate, in particular, tubing, in a desirable manner.
In particular, when a tubing which is based on cellulose hydrate and is coated on the inside, is used as an artificial sausage casing, the extensive shrinkage and the associated densification and embrittlement of the material are significant defects.